1. Field of the Invention
This invention relates to a mold for molding an optical disc such as a compact disc and a laser disc.
2. Prior Art
In recent years, there have been increasingly used compact discs which store digital audio data and reproduce the recorded sound with high accuracy. As is well known in the art, a large number of fine pits are formed in a selected pattern in a surface of the compact disc so as to produce digital audio signals when they are detected, thereby reproducing the recorded music or other sounds. Generally, such an optical disc is injection molded using a disc-shaped thin stamper secured at one face to a mold and having a large number of fine projections corresponding in shape and size to the pits of the resultant optical disc. One such a conventional mold 10a as shown in FIG. 1 comprises a stationary mold member 12a having a recess 16a, and a movable mold member 14a movable toward and away from the stationary mold member 12a. The movable mold member 14a has a projection 22a formed on one face 24a thereof and adapted to be fitted in the recess 16a of the stationary mold member 12a when the two mold members 12a and 14a are mated together. The projection 22a has a flat seat surface 32a machined to a mirror surface. With this construction, when the stationary and movable mold members 12a and 14a are mated together, a mold cavity 30a is defined by the surface of the recess 16a of the stationary mold member 12a and the flat seat surface 32a as shown in FIG. 1. A disc-shaped stamper 34a having a thickness of about 0.3 mm is detachably secured to the seat surface 32a of the projection 22a by a bolt (not shown) received in a central hole 26a of the movable mold member 14a. A molding material is injected from a injection molding machine 36 into the mold cavity 30a via a nozzle 38a mounted on the stationary mold member 12a so as to mold a compact disc. The stationary and movable mold members 12a and 14a are made of steel such as one manufactured and sold by Uddeholm under the tradename of Stavax. Holes 40a and 42a fare formed in the stationary and movable mold members 12a and 14a, respectively, for passing cooling water therethrough.
Usually, various kinds of stampers are provided for being exchangeably mounted on the flat seat surface 32a of the movable mold member 14a so as to produce various kinds of compact discs. With this conventional mold 10a, however, if a jig or other tool inadvertently hits against the flat seat surface 32a of the movable mold member 14a at the time of exchaning the stampers 34a and at other occasions, the seat surface 32a is damaged. As a result, the thin stamper 34aseated on the damaged seat surface 32a is deformed at the damaged portion of the seat surface 32a, so that such deformation is transferred to the molded optical discs. Thus, there is a possibility that the optical discs of an inferior quality are produced in a large quantity. And, when the flat seat surface 32a of the movable mold member 14a is damaged, the damage seat surface 32a must be polished, which leads to low productivity.
In addition, when the molding resin is injected into the mold cavity 30a, the stamper 34a is subjected to vibration by the molding resin under pressure, so that the marginal portion of the flat seat surface 32a tends to undergo damage. This is undesirable since the stamper 34a is not held in proper position with respect to the flat seat surface 32a.